Boreholes, which are also commonly referred to as “wellbores” and “drill holes,” are created for a variety of purposes, including exploratory drilling for locating underground deposits of different natural resources, mining operations for extracting such deposits, and construction projects for installing underground utilities. A common misconception is that all boreholes are vertically aligned with the drilling rig; however, many applications require the drilling of boreholes with vertically deviated and horizontal geometries. A well-known technique employed for drilling horizontal, vertically deviated, and other complex boreholes is directional drilling. Directional drilling is generally typified as a process of boring a hole which is characterized in that at least a portion of the course of the bore hole in the earth is in a direction other than strictly vertical—i.e., the axes make an angle with a vertical plane (known as “vertical deviation”), and are directed in an azimuth plane.
Conventional directional boring techniques traditionally operate from a boring device that pushes or steers a series of connected drill pipes with a directable drill bit at the distal end thereof to achieve the borehole geometry. In the exploration and recovery of subsurface hydrocarbon deposits, such as petroleum and natural gas, the directional borehole is typically drilled with a rotatable drill bit that is attached to one end of a bottom hole assembly or “BHA.” A steerable BHA can include, for example, a positive displacement motor (PDM) or “mud motor,” drill collars, reamers, shocks, and underreaming tools to enlarge the wellbore. A stabilizer may be attached to the BHA to control the bending of the BHA to direct the bit in the desired direction (inclination and azimuth). The BHA, in turn, is attached to the bottom of a tubing assembly, often comprising jointed pipe or relatively flexible “spoolable” tubing, also known as “coiled tubing.” This directional drilling system—i.e., the operatively interconnected tubing, drill bit, and BHA—can be referred to as a “drill string.” When jointed pipe is utilized in the drill string, the drill bit can be rotated by rotating the jointed pipe from the surface, through the operation of the mud motor contained in the BHA, or both. In contrast, drill strings which employ coiled tubing generally rotate the drill bit via the mud motor in the BHA.
Advances in drilling techniques and technology have produced various types of downhole tools that provide an assortment of enhanced drilling features, such as hole enlargement, steering feedback, torque reduction, BHA monitoring, borehole evaluation, and drag resistance improvement. A few examples of some such downhole tools can include rotary steerable tools, stabilizers, sensor assemblies, agitator tools, reamers, measurement-while-drilling (MWD) tools, etc. On the larger end of the spectrum, some electric motors are used for rotating the drill bit and some for operating downhole pumps to provide forward and reverse circulation of the drilling fluid.
With the installation of downhole tools comes a need for dependable and efficient power sources to drive and regulate the hardware. One conventional way to provide electricity to downhole hardware is via a power cable that transmits electrical current from the surface, downhole through the drill string, and to the tool. In some other assemblies, high-temperature high-capacity batteries have been used as a local source of power for downhole electrical devices. As an alternate means of power generation, fuel cells have been used which generate electricity from an electrochemical reaction between petroleum products in the drilling fluid and an oxidant in a cell. In addition, a variety of mud-driven electrical power generators have been devised for supplying electricity to downhole tools. Although many downhole tools are powered by electricity, there are various tools that are fluid-driven or mechanically-driven devices that cannot be powered by these prior art approaches.